1. Field of the Invention
This invention relates to linear dipole antennas and, more particularly, to an end-fed sleeve dipole antenna with improved impedance match, increased bandwidth and simplified mechanical design.
2. Description of the Related Art
The following descriptions and examples are given as background only.
Linear dipole antennas are often formed by coupling two ¼-wavelength conductors, or radiative elements, back to back for a total length of λfs/2, where λfs is the free space wavelength of the antenna radiation. Dipoles whose total length is one-half the wavelength of the radiated signal are called ½-wave dipoles, and in many cases, the term “dipole” is synonymous with “½-wave dipole.” The radiation resistance of an ideal ½-wave dipole is approximately 73 Ohms (if wire diameter is ignored), and the maximum theoretical directivity of the ideal ½-wave dipole is 1.64, or 2.15 dBi. However, the actual gain may be a little less due to ohmic losses.
There are generally two types of linear dipole antennas: center-fed and end-fed dipoles. In center-fed dipole 100 of FIG. 1, the radiative elements 110/120 are arranged back-to-back and are fed at the center-point or “feed point” 130 of the dipole by a feed transmission line 140 extending away from the dipole in a direction perpendicular to the dipole axis (i.e., the longitudinal axis of the dipole extending through the radiative elements). Balun 150 is coupled to the feed point to effect a transformation from the balanced (symmetric) feed point to the unbalanced (e.g. coaxial) transmission line, and in some cases, to match the feed point impedance to the characteristic impedance of the coaxial feed transmission line.
Similar to the center-fed dipole, end-fed dipole 200 of FIG. 2 generally comprises a pair of radiating elements 210/220, which are driven at center feed point 230 via internal feed transmission line 240. However, the end-fed dipole differs from the center-fed dipole in that it is driven from one end by routing the (internal) feed transmission line along the dipole axis. This prevents the feed transmission line from interfering with the antenna radiation pattern in the H-plane, thus enabling the end-fed dipole to produce a nearly perfect isotropic radiation pattern in the H-plane. However, it is generally necessary to employ some sort of “choke” 250 at lower radiating element 220 of the end-fed dipole to prevent the antenna current from inducing common mode currents on the exterior of the feed transmission line and distorting the E-plane pattern. Pattern distortions caused by common mode currents are discussed in more detail below with reference to FIGS. 3A-C.
By definition, the E-plane of an antenna is the plane containing the far-field electric field and the direction of maximum radiation. Thus, for an electric dipole and a linear dipole, the E-plane contains the axis of the antenna. Since the ideal linear dipole is rotationally symmetric about its axis, the E-plane definition really describes one of an infinite number of planes containing the dipole axis. By corollary, the H-plane is the plane perpendicular to the dipole axis.
FIG. 3A shows 3-dimensional radiation pattern 300 of an ideal ½-wave linear dipole at its ½-wave (fundamental) resonance. As shown in FIG. 3A, an ideal antenna will exhibit a near perfect isotropic radiation pattern in the H-plane and a directional pattern in the E-plane. An isotropic H-plane pattern is desirable in many metrology applications, including Over-The-Air (OTA) testing of mobile telephones and other devices.
However, any “real” dipole, which is fed by a single-ended transmission line (such as a coaxial cable) or even a balanced transmission line, will suffer at least some performance deviation or degradation from the idealized pattern (shown in FIG. 3A), due to common mode currents flowing from the antenna onto the exterior of the feed transmission line or electromagnetic coupling of the near or far fields directly to the line. For instance, while the end-fed dipole exhibits very good isotropy in the H-plane, it demonstrates significant E-plane pattern distortion when the dipole is fabricated without a choke (e.g., choke 250 of FIG. 2). That is, without a choke in place to demarcate the lower radiating element, the end-fed dipole will induce (via conduction) common mode current on the exterior of the feed transmission line. This common mode current results in a current distribution, which is much longer than the intended ½-wavelength of the dipole, and thus, greatly perturbs the E-plane radiation pattern. If the feed transmission line is coincident with the axis of the end-fed dipole, the H-plane radiation pattern will remain isotropic no matter how much common mode coupling exists. However, poor test results may be obtained if the distortion in the E-plane pattern is great enough.
3-dimensional radiation pattern 310 of a half-wave linear dipole operating at its 3/2-wave resonance is shown in FIG. 3B to demonstrate the pattern distortion that may be caused when common mode currents are induced on the exterior of the feed line. In other words, FIG. 3B illustrates the case in which the coupling of common mode currents results in a current distribution on the feed line, which is much longer than the intended ½-wavelength. This current distribution clearly perturbs the antenna radiation pattern, as shown in the comparison of FIGS. 3A and 3B.
FIG. 3C shows 2-dimensional graph 330 comparing the E-plane patterns of a ½-wave linear dipole operating at its ½-wave (fundamental) resonance 340 and its 3/2-wave resonance 350. The E-plane pattern distortion generated by operating the dipole at its 3/2-wave resonance (FIG. 3B) is clearly illustrated in FIG. 3C. In addition to E-plane pattern distortion, FIG. 3C indicates how common mode currents can lead to a near nulling of the far fields in the H-plane. While E-plane pattern distortion is not necessarily a problem in OTA testing, the deep null produced in the H-plane (−3.3 dBi gain, FIG. 3C) results in very poor quality OTA test measurements and should be avoided.
In order to avoid pattern distortion caused by common mode currents, it is often desirable to employ some sort of “choke” 250 on end-fed dipole 200 (FIG. 2) to demarcate lower radiating element 220 and to “choke off” or prevent the antenna current from flowing along the exterior of the feed transmission line. At sufficiently low frequencies (e.g., frequencies up to about 100 MHz), ferrite choke beads may be coupled to the feed transmission line of an end-fed dipole to choke off the common mode current induced by the dipole. However, ferrite choke beads are not typically used at significantly higher frequencies, such as ultra high frequencies (UHF) and above, since they are typically very lossy at these frequencies and greatly reduce the radiation efficiency of the antenna. In addition, as ferrite choke beads cannot provide a high choking impedance at such high frequencies, they fail to prevent common mode current from flowing on the exterior of the feed transmission line.
Another common approach for reducing pattern distortion is to employ a ¼-wave choke sleeve. The most abstract description of a “sleeve,” in the context of linear antennas, is that the skin effect will prevent penetration of electromagnetic fields into a good conductor. Thus, a conducting sleeve can support two independent current distributions: one on its interior surface and one on its exterior surface. While somewhat limited in bandwidth, the ¼-wave choke sleeve is intrinsically low loss and can provide an extremely high choking impedance near its ¼-wave resonance frequency.
Conventional end-fed dipole 400 employing a ¼-wave choke sleeve is shown in the cross-sectional diagram of FIG. 4A. In the conventional end-fed sleeve dipole, feed transmission line 450 is routed through one half of dipole 410 and coupled to the other half of dipole 420 at feed region 440 of the antenna. The feed transmission line typically comprises a semi-rigid (or rigid) coaxial cable having 50 Ohms characteristic impedance. Dielectric support 430 is provided at feed region 440 to physically separate and electrically isolate the two radiative elements of the dipole. Dielectric spacer 435 is provided at the lower end of the dipole to ensure that feed transmission line 450 is arranged concentrically within the left half of the dipole.
At feed region 440, the outer conductor (or shield) of coaxial feed transmission line 450 is electrically connected to the left half of the dipole (e.g., by soldering the outer conductor to the left dipole element). This connection also establishes a short circuit inside the choke near feed region 440, while dielectric spacer 435 maintains concentricity, thus, preventing an inadvertent short at the lower end of the dipole. The center conductor of coaxial feed transmission line 450 passes through dielectric support 430 and is connected to the right half of the dipole (again, by soldering the center conductor to the end of the right dipole element). The free end of the coaxial feed transmission line is coupled to coaxial input connector 460 for connection to a source.
In FIG. 4A, the exterior surface of the left half of dipole 410 serves as the radiating element, while the interior surface serves as the outer conductor of the choke sleeve. The exterior surface of the portion of coaxial feed transmission line 450 extending through the left half of the dipole serves as the inner conductor of the choke sleeve. It is generally desirable to make the diameter of the dipole element large compared to the external diameter of the coaxial feed line. This increases the characteristic impedance of the choke sleeve, and thus, the effectiveness of the choke.
In a half-wave dipole, the left half of the dipole (comprising the first radiating element and the choke sleeve) and the right half of the dipole (comprising the dielectric support and the second radiating element) are each formed to be approximately λfs/4 in length, where λfs is the free-space wavelength of the dipole radiation. A choke sleeve, which is λfs/4 in length, is referred to as a “¼-wave choke sleeve.”
A ¼-wave choke sleeve exploits the impedance transformation of a uniform transmission line to transform a short circuit (at the feed region 440) to an open circuit, which is placed between the lower end of the dipole and the exterior of the feed transmission line (e.g., at dielectric spacer 435). This transformation allows the ¼-wave choke sleeve to effectively choke the current at the bottom of the choke sleeve at the ¼-wave (resonant) frequency. However, near field coupling of the electric field to the exterior of the feed transmission line still exists, even if the current is reduced to zero at the bottom of the choke sleeve. In addition, the choke acts as an inductance connecting the lower end of the dipole to the exterior of the coaxial feed transmission line below its ¼-wave frequency. Because of these two coupling mechanisms, the ¼-wave choke sleeve is not entirely effective, as it cannot completely eliminate common mode currents on the exterior of the coaxial feed transmission line.
In some cases, performance may be improved by utilizing two or more ¼-wave choke sleeves followed by one or more ferrite choke beads. For example, and as shown in end-fed dipole 400′ of FIG. 4B, two ¼-wave choke sleeves 410/470 may be used to increase choking impedance, while ferrite beads 480 are coupled behind the chokes to reduce coupling of the near electric field to the exterior of coaxial feed transmission line 450. While this may slightly improve performance over the embodiment shown in FIG. 4A, the overall performance of the dipole antenna shown in FIG. 4B is still limited by poor impedance match and narrow bandwidth.
Therefore, a need exists for an improved end-fed sleeve dipole, and more specifically, an end-fed sleeve dipole with improved impedance match and increased bandwidth that exhibits an E-plane pattern that is similar to the pattern of an ideal half-wave dipole. In addition to performance, it is also desirable to provide a dipole antenna that maintains a simple mechanical design, as a difficult mechanical design generally results in a manufactured product with reduced reliability and great variation from unit to unit.